Semipermeable membrane

Scheme of semipermeable membrane during hemodialysis, where red is blood, blue is the dialysing fluid, and yellow is the membrane.

A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane which will allow certain molecules or ions to pass through it by diffusion and occasionally specialised "facilitated diffusion". The rate of passage depends on the pressure, concentration and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute.

Depending on the membrane and the solute, permeability may depend on solute size, solubility properties, or chemistry. An example of a semi-permeable membrane is a lipid bilayer, on which is based the plasma membrane that surrounds all biological cells. Many natural and synthetic materials thicker than a membrane are also semipermeable. An example of this is the thin film on the inside of an egg.

A semipermeable membrane is consisted of a phospholipid bilayer, essentially meaning that a group of phospolipids (consisting of a phosphate head and 2 fatty-acid tails), arrange themselves into a double-layer, with the hydrophilic phosphate heads exposed to the water content outside and within the cell, while the hydrophobic fatty-acid tails hide in the inside. Protein channels float through the phospholipids, and collectively, this model is known as the Fluid Mosaic model.

Another example of a semipermeable membrane is visking tubing.

See also

• ADME
• Osmosis
• Reverse osmosis